People who drink more than one fizzy soft drink a day are “up to 60 per cent more likely to develop obesity and high blood pressure, leading to [heart] attacks and strokes”...

Diet soft drinks may be just as bad for you as those containing sugar, the Daily Mail reported. People who drink more than one fizzy soft drink a day are “up to 60 per cent more likely to develop obesity and high blood pressure, leading to [heart] attacks and strokes” the Mail said. Obesity and high blood pressure are among a number of unfavourable metabolic characteristics known to increase the risk of cardiovascular disease; these are sometimes referred to in combination as the ‘metabolic syndrome’.

The newspaper reported that researchers found that the risk of metabolic syndrome was also increased in people drinking diet soft drinks, a finding that surprised researchers, who have suggested a number of explanations, including that people who drank diet drinks may have more unhealthy eating habits in general.

The research shows a link between fizzy soft drink consumption, including diet drinks, and metabolic disease risk. The study authors themselves point out that this association does not mean that soft drinks cause metabolic syndrome, and that further research is needed to explain why diet drinks might have similar effects to standard soft drinks.

Where did the story come from?

The study was conducted by Ravi Dhingra, Ramachandran Vasan and colleagues in Boston, Massachusetts, USA. The study was funded by the National Institutes of Health, National Heart, Lung, and Blood Institute, and the American Diabetes Association. It was published in the peer-reviewed medical journal Circulation, which is the journal of the American Heart Association.

What kind of scientific study was this?

This paper reports one analysis of a large cohort study (the Framingham Offspring study) which looks at the association between soft drink consumption on the risk of metabolic syndrome.

In this study, metabolic syndrome was defined as three or more of: a greater waist circumference; raised blood sugar levels; raised blood pressure; higher levels of unhealthy fats (triglycerides); or reduced levels of healthy fats (high density lipoprotein) in the blood.

In the main part of this study about 5,000 adults (aged about 51-56 on average) who did not have metabolic syndrome were asked how many 12oz soft drinks they drank a day on average, and what type of soft drinks they were (diet or regular). They were then followed for an average of four years to see if they developed metabolic syndrome.

The risk of developing metabolic syndrome was compared for people who drank at least one soft drink a day on average and people who drank less than one soft drink a day on average. The researchers used complex statistical methods to take into account differences between the groups, including other factors that could affect the results (age, sex, level of physical activity, smoking and the type of diet people ate).

What were the results of the study?

The study found that, over four years, the odds of developing metabolic syndrome were increased by 44% in people who drank at least one soft drink a day compared with people who drank less than one soft drink a day on average. When analysed by type of soft drinks, people who drank at least one diet soft drink a day were more likely to develop metabolic syndrome than those who drank less than one regular or diet soft drink a day on average.

What interpretations did the researchers draw from these results?

The authors concluded that drinking soft drinks is associated with a higher risk of metabolic syndrome in middle-aged adults. This may, in turn, increase risk of adverse cardiovascular events.

What does the NHS Knowledge Service make of this study?

This study, which was published in a journal with a peer-review process. Though this means that the methods were checked there are some limitations, which the authors acknowledge:

Although the authors did adjust for factors that could affect the results, such as diet, there could be other unknown factors that might account for the increase in metabolic syndrome seen in people who drank soft drinks. Therefore we cannot say for certain that drinking soft drinks causes metabolic syndrome;

Although the odds of having metabolic syndrome were increased by 44%, the absolute increase in the proportion of people who developed metabolic syndrome was not very large, at 3%;

Different definitions of metabolic syndrome exist, and this study used the US National Cholesterol Education Program definition. It is not known how results would have differed if different criteria, for example the World Health Organisation suggested criteria, were used;

This study included only white Americans; the effects of soft drink consumption might differ in different population groups.

Although the individual features of the metabolic syndrome do increase the risk of heart disease, not all people who have metabolic syndrome will go on to develop heart disease. The effects of soft drink consumption on heart disease remain to be determined.

This type of study can highlight interesting links and associations, but is not designed to test whether one risk factor causes another, or in this case a cluster of risk factors. A single dietary cause for the metabolic syndrome is unlikely.

Sir Muir Gray adds…

These results are puzzling and will stimulate other researchers to examine this issue; often more than one study is needed to clarify the answer.

This research should not be interpreted as justifying or requiring that you should give up diet drinks for drinks containing glucose.